The phenomenon commonly known as “Red Tide” is scientifically referred to as a harmful algal bloom (HAB), which occurs when a microscopic algae species multiplies rapidly in coastal waters. Human health impacts are generally temporary and arise from exposure to toxins produced by these organisms, most frequently Karenia brevis in the Gulf of Mexico and along the US Atlantic coast. These blooms can discolor the water, though not all HABs are visible. Health effects relate mainly to two forms of exposure: breathing aerosolized toxins or ingesting contaminated seafood.
How Red Tide Toxins Affect the Human Body
The primary culprit in US red tides is the dinoflagellate Karenia brevis, which produces potent neurotoxins called brevetoxins. Brevetoxins are fat-soluble molecules that interfere with the function of nerve cells. These toxins bind to voltage-gated sodium channels (VSSCs) in nerve cell membranes. This binding causes the sodium channels to remain open longer than normal, disrupting neurological processes.
Humans are exposed to brevetoxins through three main routes: inhalation, direct contact, and ingestion. When waves break, the toxins become aerosolized into tiny droplets that can be inhaled. Direct exposure can occur through skin or eye contact while swimming. Ingestion of contaminated shellfish is the most severe route, leading to Neurotoxic Shellfish Poisoning (NSP).
Common Symptoms and Typical Duration
The duration of red tide symptoms depends heavily on the route and level of exposure. The most common and mildest symptoms result from inhaling aerosolized toxins near the beach. These symptoms include coughing, sneezing, throat irritation, and a burning sensation in the eyes.
For the general population, these respiratory and contact symptoms are self-limiting and resolve quickly. Relief usually occurs within a few hours of leaving the affected area and moving to an air-conditioned space. Skin irritation or a mild rash from water contact subsides shortly after washing off with fresh water.
Ingestion of contaminated shellfish, such as oysters, clams, or mussels, causes Neurotoxic Shellfish Poisoning (NSP). Symptoms of NSP appear rapidly, from a few minutes to 24 hours after consumption. Initial complaints often include gastrointestinal issues like nausea, vomiting, and diarrhea, followed by neurological effects.
Neurological symptoms of NSP include tingling and numbness, particularly around the mouth and extremities, and a reversal of hot and cold sensations. While the illness is self-limited, the duration is longer than simple inhalation exposure, lasting from a few hours to several days. Fatalities are uncommon due to vigilant shellfish monitoring programs.
When to Seek Medical Attention
Most symptoms from red tide exposure are temporary and resolve on their own, but certain signs indicate the need for medical evaluation. Seek care if mild respiratory symptoms, such as persistent coughing or throat irritation, do not improve within 24 to 48 hours of leaving the bloom area. This is important if symptoms worsen instead of getting better.
Individuals with pre-existing chronic respiratory conditions, such as asthma, emphysema, or COPD, should be cautious. For this high-risk group, red tide toxins can trigger a severe respiratory inflammatory response, requiring immediate medical attention upon symptom onset. Difficulty breathing, chest tightness, or wheezing not relieved by usual prescribed medication warrants an urgent visit.
If ingestion of contaminated shellfish is suspected, medical help should be sought immediately due to the risk of Neurotoxic Shellfish Poisoning. Immediate attention is required for severe or prolonged neurological symptoms, such as dizziness, loss of coordination, or slurred speech. Persistent vomiting or diarrhea leading to dehydration, such as excessive thirst or reduced urination, requires prompt medical management.